refactor: one Go module; brain search in bin/brain + internal/brain.

Collapse nested kbsearch/chats go.mod into the root module. Ranking stays
cgo-free under internal/brain/rank so CI does not need ladybug. bin/kb/search
is a deprecation wrapper that still sets CGO and builds the binary.

internal/brain/yaml.go

@@ -0,0 +1,107 @@
+// YAML emitter ported from bin/kb/yamlout.py — preserves insertion order.
+package brain
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+// KV is an ordered key-value pair for maps.
+type KV struct {
+	K string
+	V any
+}
+
+// Dict is an ordered map (slice of KV).
+type Dict []KV
+
+func toYAML(node any, indent int) string {
+	pad := strings.Repeat("  ", indent)
+	switch n := node.(type) {
+	case Dict:
+		if len(n) == 0 {
+			return pad + "{}\n"
+		}
+		var b strings.Builder
+		for _, kv := range n {
+			switch nv := kv.V.(type) {
+			case Dict:
+				if len(nv) == 0 {
+					b.WriteString(pad + kv.K + ": {}\n")
+				} else {
+					b.WriteString(pad + kv.K + ":\n" + toYAML(nv, indent+1))
+				}
+			case []any:
+				if len(nv) == 0 {
+					b.WriteString(pad + kv.K + ": []\n")
+				} else {
+					b.WriteString(pad + kv.K + ":\n" + toYAML(nv, indent+1))
+				}
+			default:
+				b.WriteString(pad + kv.K + ": " + scalar(nv) + "\n")
+			}
+		}
+		return b.String()
+
+	case []any:
+		if len(n) == 0 {
+			return pad + "[]\n"
+		}
+		var b strings.Builder
+		for _, item := range n {
+			if d, ok := item.(Dict); ok {
+				b.WriteString(pad + "-\n" + toYAML(d, indent+1))
+			} else {
+				b.WriteString(pad + "- " + scalar(item) + "\n")
+			}
+		}
+		return b.String()
+
+	default:
+		return pad + scalar(node) + "\n"
+	}
+}
+
+func scalar(v any) string {
+	switch t := v.(type) {
+	case nil:
+		return "null"
+	case bool:
+		if t {
+			return "true"
+		}
+		return "false"
+	case int:
+		return strconv.Itoa(t)
+	case int64:
+		return strconv.FormatInt(t, 10)
+	case float64:
+		return fmtFloat(t)
+	case float32:
+		return fmtFloat(float64(t))
+	case string:
+		return quoteIfNeeded(t)
+	default:
+		// fallback
+		return fmt.Sprintf("%v", v)
+	}
+}
+
+func fmtFloat(f float64) string {
+	s := strconv.FormatFloat(f, 'g', -1, 64)
+	if !strings.ContainsAny(s, ".eE") {
+		s += ".0"
+	}
+	return s
+}
+
+func quoteIfNeeded(s string) string {
+	if strings.Contains(s, "\n") {
+		return strconv.Quote(s)
+	}
+	if s == "" || strings.ContainsAny(s, ":#'\"[]{}&*!|>%@`") || s != strings.TrimSpace(s) {
+		return strconv.Quote(s)
+	}
+	return s
+}